In a wireless communication system, a receiver such as a mobile terminal communicates with a transmitter such as a base station over a wireless channel. The quality of the wireless channel may be reduced due to interference arising from various sources e.g. interfering transmissions from other transmitters, the interaction of transmitted signals with the physical environment in which they propagate, RF imperfection at transmitter or receiver side, etc.
In an 3GPP Long Term Evolution (LTE) system, the receiver may be a user equipment (UE) and the transmitter may be an evolved Node B (eNB) transmitting within a cell site (cell). Data are grouped together into transport blocks, and modulated and coded according to a modulation and coding scheme (MCS) selected from a number of possible MCSs.
Intra-cell interference at the UE may arise as a result of signals transmitted within that cell serving the UE, and inter-cell interference may arise due to signals transmitted in other cells in the vicinity of the UE.
Both the UE and the eNB may comprise multiple antennae for improved spatial diversity, transmit or receive beamforming, spatial multiplexing or more generally for multiple-input multiple-output (MIMO) transmission.
To date, most of the efforts in 3GPP LTE standardization have been spent on mitigating interference at the transmitter side, e.g. through spatial multi-antenna precoding, whereby data is transmitted from and received by multiple collocated antennae, coordinated beamforming (BF) to effect directional transmission/reception to direct interfering signals away from the UE, inter-cell interference coordination (eICIC, feICIC), and coordinated multi-point transmission (COMP). In CoMP, transmission from multiple antennae is coordinated (as in MIMO), although these multiple antennae are not necessarily collocated—e.g. transmission across multiple cells or transmission points may be coordinated to reduce interference and/to make desired data available to a UE from multiple cells contemporaneously (that is, in CoMP, a UE is not necessarily restricted to receiving desired data from only the serving cells or transmission points and may also be able to receive this data from neighboring cells or neighboring transmission points). CoMP defines transmission points (TPs) as a collection of one or more collocated antennae and, in general, transmissions across multiple TPs may be coordinated.
Transmitter-side interference mitigation techniques typically rely on the UE feeding back accurate information about the channel conditions it is currently experiencing e.g. in the form of channel state information (CSI). The transmitter is then able to adapt its various transmission procedures (e.g. by selecting an appropriate MCS) and/or control or influence the behaviors of other transmitters based on the knowledge of the reported conditions. Channel state information may be determined and fed back with an aim of achieving a particular target transport block error rate (BLER) for example of 10%.
In LTE Release-11, linear interference suppression (IS) at the receiver side was investigated, and the observed performance gains motivated the introduction to TS 36.101 [3] of improved minimum performance requirements for UE demodulation under spatially colored interference. The Network Assisted Interference Cancellation and Suppression (NAICS) studies in 3GPP [2] and the recently approved work item [3] take another step towards further enhanced UE receivers targeting intra-/inter-cell interference cancellation (IC), most likely based on non-linear receiver processing such as e.g. maximum likelihood (ML) detection or symbol level interference cancellation (SLIC).
Enhanced user equipment (UE) receivers have also been studied in 3GPP standardization under the LTE Release-12 study item on NAICS [1]. The study was mainly motivated by the fact that dense wireless network deployments together with the growth of data traffic will lead to interference limited scenarios. Co-channel interference, either from inter-cell or co-scheduled intra-cell users, is expected to become the dominant factor limiting the overall downlink throughput performance of wireless networks.